The present invention concerns a novel transmission system for differential speed control and, more particularly, a narrow range draw transmission that is suitable for servo control of rotating mechanisms requiring differential speed control up to the 10 percent range.
Differential control of draw percentage is required during the processing of continuous strands or webs of a generally elastic substrate. Such differential control is utilized in the areas of the process line where it is essential to maintain tension or to attain and maintain a constant length, such as in the drawing, printing, coating or perforating of a paper substrate. In closed loop applications, the substrate is monitored at various points throughout the process line and the differential draw is adjusted to compensate for changes in elasticity.
In the prior art, variable draw was obtained by either (a) using a wide range device over a small portion of its total range capability, or (b) using a wide range device in conjunction with a mechanical differential transmission.
Wide-range devices which were used over a small portion of their total range capability included variable pitch "V" pulleys in conjunction with standard or specially formed "V" belts, variable pitch toothed pulleys used in conjunction with a special "profile conforming" chain, friction drives using balls or cones or discs, variable voltage DC drives, variable frequency AC drives, etc. Generally, these wide-range devices have a control range between 4:1 and 20:1. This creates extensive range capability problems when the device is used as the output of a narrow band servo system. Many of these devices are subject to localized wear seriously limiting their useful life when they are continuously used over a narrow portion of their normal range.
As stated above, variable draw was also obtained in the prior art by utilizing the above devices in conjunction with a mechanical differential transmission. This could be an epicyclic gear arrangement, a harmonic drive set, a bevel or spur gear differential, or other related arrangements. While this approach reduces the total range of the device, it does not appreciably reduce wear grooving. Further, adding the mechanical differential system increases expense and adds to the complexity of the total machine.